SHELL MODELS MODELOS DE CAPAS FUNCIONES DE ONDA WAVE FUNCTIONS A Reformulation of the Mode-Goupling Method CARBON 14 REACTIONS CHARGE-EXCHANGE REACTIONS CLUSTERS COLLECTIVE MODEL CROSS SECTIONS Cluster of nucleons as elementary modes of excitation in nuclei Continuum DWBA ESPACIO DE HILBERT ESTADOS FUNDAMENTALES EXCITED STATES GROUND STATES Graphical multistep shell-model calculation of the even tin and lead ground states HILBERT SPACE LEAD 208 LEAD ISOTOPE MODELO DEL CUARTETO Multi-step shell-model method. The nuclei 204pb and 202pb Multi-step shell-model treatment of six-particle systems OXYGEN 14 PARTICLE STRUCTURE PARTICULAS POLONIUM 208 Pair exchanqe reactions QUARTET MODEL